Microwave moisture sensing system including means to continuously change the transmission path of the microwave energy

ABSTRACT

An apparatus for measuring the amount of moisture in a sample which is generally formed of a microwave energy source, a microwave radiating element connected to the source, a microwave energy receiving element, and a microwave energy indication connected to the receiving element. The sample is positioned between the radiating and receiving elements and a structure is provided for changing the energy transmission path through the sample by at least one-half wavelength of the microwave energy. Such means may take the form of a frequency modulator connected to the source, a dielectric disc in the transmission path, or a structure for continuously changing the distance between the radiating element and the sample or combination of these means.

United States Patent [72] Inventors Leroy H. Busker Rockton, 11].;Robert J. Mosher, Beloit, Wis.

[21] Appl. No. 679,325 v [22] Filed Oct. 31, 1967 [45] Patented Aug. 10,1971 [73] Assignee Belolt Corporation Belolt, Wis.

[54] MICROWAVE MOISTURE SENSING SYSTEM INCLUDING MEANS TO CONTINUOUSLYCHANGE THE TRANSMISSION PATH OF THE MICROWAVE ENERGY 3 Claims, 16Drawing Figs.

[50] Field of Search 324/58,

[5 6] Reierences Cited UNITED STATES PATENTS W 3,025,463 3/1962 Luoma eta1. 3 24/585 (B) 2/1963 Walker 324/58 (A) X 324/58.5 (13) 3,079,5512,912,643 11/1959 Rohrbaugh etal.

ABSTRACT: An apparatus for measuring the amount of moisture in a samplewhich is generally formed of a microwave energy source, a microwaveradiating element connected to the source, a microwave energy receivingelement, and a microwave energy indication connected to the receivingelement. The sample is positioned between the radiating and receivingelements and a structure is provided for changing the energytransmission path through the sample by at least onehalf wavelength ofthe microwave energy. Such means may take the form of a frequencymodulator connected to the source, a dielectric disc in the transmissionpath, or a structure for continuously changing the distance between theradiating element and the sample or combination of these means.

#WZGIPATOQ MICROWAVE MOISTURE SENSING SYSTEM INCLUDING MEANS TOCONTINUOUSLY CHANGE THE TRANSMISSION PATH OF THE MICROWAVE ENERGYBACKGROUND OF THE INVENTION I 1. Field of the Invention This inventionrelates generally to a method of an apparatus for sensing constituent ofa material which exhibits'an influence on microwave energy and moreparticularly to a method of an apparatus for measuring the amount ofmoisture in a material. The present invention has specific applicabilityin the measurement of moisture content in paper, paper products, pulpslurries, and the like materials. In general, however, the presentinvention can be employed to measure the moisture content of anymaterial, whether liquid, semiliquid, or solid.

2. Description of the Prior Art The method for measuring moisturecontent of material which consists of placing a sample between twomicrowave horns which are connected to a microwave generator and amicrowave detector is well known in the priorart. However, thistechnique is applicable only when the position of the sample between thehorns and the distance between the horns can be accurately maintained.It has been found that there is a drastic effect upon the moisturesignal due to the position of the sample with respect to the microwavehorns.

It is well established that a dielectric sample placed between amicrowave sending and receiving horn will establish a standing wave dueto reflection of energy from the faces of the dielectric. If thereceiving horn is assumed to have no reflec tion, then without thesample present, no standing wave exists. However, when the sample isplaced between the horns, a standing wave is established which isdependent upon the amount of reflected energy from the surface and theposition of the sample. 1

When the moisture content of a paper web, for example, is beingmeasured, the position of the web cannot be maintained fixed withrespect to the horns without a considerable amount of difficulty beingencountered. In the measurement of moisture of a paper web, it isnecessary to scan across the paper web to determine the moisturecontent. of the entire width thereof. Such a scanning of the paper web,which requires either movement of the horns, or movement of the web withrespect to the horns, usually results in some change in the distancebetween the horns and the web. As a result of such a change, thestanding wave which exists will affect the amount of energy which isreflected from the surface of the sample. Since the measurement of suchreflected energy or the measurement of the transmitted energy isemployed in the determination of the moisture content within the sample,such a change of position of the sample will result in erroneousreadings indicative of the moisture content.

In the manufacture of paper the moisture content of a paper web must bedetermined during the manufacturing process. That is, when the paper webis being dried, for instance, it may be desirable to determine themoisture content therein. Since such drying of the paper web is effectedon drying rolls, the measurement of the moisture content mustnecessarily be effected at some point intermediate the drying rollswhere the measuring apparatus can be easily inserted and mounted withrespect to the traveling paper web. However, a traveling paper web at apoint intermediate a pair of drying rolls normally has a component ofmovement transverse to the plane thereof. That is, between the dryingrolls, a traveling paper web may flutter". Such movement of the paperweb would be towards and away from the microwave horns positioned onopposite side thereof employed for measuring the moisture contentthereof. Such movement with respect to the microwave horns, as discussedabove, would produce erroneous readings with respect to the moisturecontent thereof. Therefore, a need exists for a system which eliminatesthe position sensitivity of a sample.

SUMMARY OF THE INVENTION 7 It is, therefore, an object of the presentinvention to provide an apparatus for measuring the amount of moisturein a sample which eliminates the difiiculty of the standing waveresulting from the position of a sample.

It is another object of the present invention to provide an apparatusfor measuring the moisture content in a sample which eliminates thedifficulty of the changing reflection from the sample surface as itsspacing between the microwave horns changes.

These and other objects are realized by the structure of the presentinvention which generally includes a microwave energy radiating elementpositioned to impinge microwave energy on the sample, a microwave energyreceiving element positioned to receive microwave energy transmittedthrough the sample, and means for continuously changing the transmissionpath from the radiating element to the sample as measured in wavelengths of the microwave energy at least one-half wave length of themicrowave energy.

A specific feature of the present invention resides in the provision ofa frequency modulator connected to a source of microwave energy, whichmodulator sweeps the frequency of the source through a predeterminedrange which effectively changes the transmission path from the radiatingelement to the sample at least one-half wave length of the microwaveenergy.

Another feature of the present invention resides in the provision ofmeans for continuously changing the physical distance between theradiating element and the sample, with the change in the physicaldistance being sufficient to provide a change of at least one-half wavelength in the transmission path of the microwave energy therebetween.

Still another feature of the present invention resides in the provisionof a dielectric disc of varying thickness mounted between the radiatingelement and the sample and means for rotating'the disc at apredetermined rate to present a continuously changing transmission paththrough the disc to the BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is adiagrammatic view of an apparatus for measuring the amount of moisturein a sample and illustrates the transmission of energy to and from thesample;

FIG. 2 is a diagrammatic showing of an apparatus for measuring theamount of moisture in a sample and constructed in accordance with theprinciples of the present invention;

FIG. 3 is a diagrammatic showing of another embodiment of the presentinvention for measuring the amount of moisture in a sample;

FIG. 4 is a diagrammatic showing of still another embodiment of thepresent invention;

FIG. 5 is a diagrammatic showing of another embodiment of the presentinvention;

FIG. 6 is a diagrammatic showing of an apparatus for measuring theamount of moisture in a sample which employs apparatus for amplifyingthe transmitted energy passing through the sample;

FIG. 7 is a diagrammatic showing of still another embodiment of thepresent invention for measuring the moisture content in a sample inwhich the moisture content is relatively low;

FIG. 8 is an end view of the measuring system illustrated in FIG. 7;

FIG. 9 is still another embodiment of the apparatus of such as a paperweb. The energy p, transmitted through the sample is detected orreceived by a microwave energy receiving born 18 which is connected toan indicating device 19. The reflected energy P is transmitted backtoward the trans mitting horn 16. Determination of the moisture contentof the sample 17 is provided by a measure of the amount of energy P,which is detected by the receiving horn l8.

When sample 17 is placed between the horns 16 and 18, a standing wave isestablished which is dependent upon the amount of reflected energy fromthe surface and the position of the sample with respect to the horns.Movement of the sample'17 through a quarter wavelength producesconditions of cancellation and addition of the microwave energy whichappears as'minimum and maximum reflected power. Minimums or maximums ofthe standing wave are found to appear every one-half wavelength. Becauseof the difficulty in maintaining the position of the sample fixed withrespect to the horns, the present invention contemplates a system whicheliminates the error caused by the standing wave which changes withchanges in the sample position.

In order to eliminate the problem of sample position sensitivity, thepresent invention contemplates the technique of sweeping the frequencyof the signal source 15 thus obtaining anaverage DC reading of thevarying transmitted power. If certain minimum conditions are met, theaverage reading is independent of sample position and serves as a veryuseful measurement of absorption of the microwave energy by the moisturewithin the sample 17.

The condition which must be met is that the broad band frequencymodulation of change of microwave frequency will be of sufficient changeto, produce at least a one-half wavelength change in transmission pathbetween the transmitting born 16 and the sample 17 to insure passingfrom a minimum to a maximum of reflected power. It is not necessary tobegin at any given condition of reflected power, if the change is atleast the present invention for measuring the moisture content of asample wherein the moisture content is relatively low;

FIG. is a diagrammatic showing of an apparatus for measuring themoisture content of a sample and constructed as a traveling waveresonator for overcoming the low loss encountered in relatively drypaper;

FIG. 11 is a side elevational view of an apparatus for mounting a pairof microwave horns on opposite sides of a sample and disposed forrelative movement therebetween;

FIG. 12 is a front view of the apparatus illustrated in FIG. 11;

FIG. 13 is an elevational view of a structural unit employed forhandling a slurry and which is adapted to be supported between a 'pairof microwave horns for determining the amount of moisture contenttherein; i v j FIG. 14 is a side elevational view of the structural unitillustrated in FIG. 13;

FIG. 15 is anelevational view of one of the components of a thestructural unit illustrated in Figures 13 and 14; and FIG. 16 is anelevational view of the other component of the structural unitillustratedin Figures 13 and 14 and disposed for assembly with thecomponent illustrated in Figure 15.

Like reference numerals throughout the various views of the drawings areintended to designate the same or similar Structures.

DESCRIPTION or THE PREFERRED EMBODIMENTS With reference to Figure 1 ofthe drawings, there is shown diagrammatically an apparatus for measuringthe vmoisture content of a paper web. As shown therein, a microwaveenergy source 15 is connected to a microwave energy transmitting horn 16which is disposed for transmitting microwave energy designated P, towarda sample 17, one-half wavelength in the responds to wavelengths of a l.32 cm. to 1.38 cm. To shift onehalf wavelength, the minimum distance tothe sample 17 from the horn 16 may be calculated as follows:

, (A/2)=(l.3S/2)=0.675 cm.; wherein A is the wavelength of the signal;

AA=Amax-Amin=l .38-1.32 =0.06 cm. (AFICC);

L=( t/2)'(l/A)\)=(0.675/0.06)=l1.28 cm.; wherein L is the minimumdistance between the receiving horn and the sample or the transmittinghorn and the sample.

In practice, either greater separation distance or greater sweep offrequency appears necessary to obtain a good average value. Forinstance, a frequency modulation of at least 2 or 3 gigahertz at 22gigahertz center frequency appears necessary.

It has also been found that sample position sensitivity may besignificantly reduced by the method of slanting the sample web withrespect to the microwave horns 16 and 18.

The electronic sweep of klystrons is limited to about 50 megahertz. Toobtain a minimum of one-half wavelength shift would require a separationdistance of approximately 11 feet. Therefore, the use of a klystronwould probably not prove practical. However, devices are available, suchas backward wave oscillators, that will provide a sweep of the frequencyof approximately 6 or 8 gigahertz.

One embodiment of applicant's invention for providing such a sweep ofthe source frequency is illustrated in Figure 2 of the drawings. Asshown therein, the source 15 may take the form of a microwave oscillatorwhich is controlled by a modulator 20 to provide a frequency sweep ofthe signalat the output thereof. The output of the oscillator 15 isconnected to the transmitting microwave horn 16 which transmitsmicrowave energy through the sample 17 to the receiving horn 18 The horn18 is connected to a rectifier 21 to an integrator 22. The continuallychanging frequency signal which is detected by the horn 18 is integratedby the integrator 22 to provide an average thereof as a DC signal whichis supplied to an indicating device 19. The average value of the signaldetectedby the receiving horn 18 provides a measurement of the moisturecontent within the sample 17 which is independent of the position of thesample 17.

Another embodiment of applicants invention is illustrated in Figure 3 ofthe drawing. As shown in Figure 3, the oscillator 15 provides a signalof fixed frequency to the transmitting horn 16. Disposed between thetransmitting horn 16 and the sample 17 is a dielectric chopper 23 in theform ofa wheel or disc of varying thickness made of a high dielectriclow loss material. One face 23a of the disc 23 is curved and the otherface 23b thereof is flat to present a varying thickness to the energytransmitted from the horn 16 toward the sample 17 as the disc 23 isrotated. The disc 23 is mounted on a rotatable shaft which is connectedto a motor 25 for rotation of the disc 23. Rotation of the disc 23causes a change in the transmission path length of one-half wavelengthwhich is achieved by variation of the dielectric material thicknessduring such rotation. The flat surface 23b of the disc 23 minimizes andmaintains constant the reflections to the transmitting horn 16. Thesignal detected by the receiving horn 18 is integrated by the integrator22 to provide an average of the detected signal. The average value ofsuch signal is supplied in the form of a DC voltage to the indicator l9.

Still-another embodiment of applicant's invention is illustrated inFigure 4 which employs a device for maintaining the sample 17 fixed withrespect to the horns 16 and 18. As shown therein, the source 15 ofmicrowave energy is modulated by the modulator 20, and the modulatedsignal at the output of the source 15 is connected to the transmittinghorn 16. As in the previously described embodiments, the receivinghorri' 18 is connected to the rectifier 21 'to the integrator 22, whichis in turn connected to the indicatingdevice 19. The source 15 isconnected to the horn 16 and the horn 18 is connected to the integrator22 by respective wave guides which are diagrammatically illustrated bythe single line connections in the figure. In order to maintain the hornspacing rigid, a mechanical support 26, is provided between therespective wave guides. The sample 17 is supported on a Teflon shoe 27,which is in turn supported on the receiving horn 18. In this manner, thespacing between the horns l6 and His rigidly maintained by means of themechanical support 26 and the position of the sample 17 is rigidlymaintained with respect to the horn 18.

Figure 5 illustrates another embodiment of applicants invention whichemploys a method of measurement based upon the dielectric constant(reflection) of the sample 17, rather than the loss factor (absorption)of the sample 17. At any air to dielectric interface, a reflection ofincident energy occurs. The amount of reflected energy or the reflectionfactor is a function of the dielectric constant of the material, asexpressed by the following, wherein r is the amount of reflected energyand e is the dielectric constant of the material:

As shown in Figure 5, the modulator 20 is connected to the source ofmicrowave energy and modulates the output thereof which is connected toa wave guide 16a to the transmitting horn 16. A portion of the energywhich is transmitted by the horn 16 passes through the sample 17 and isabsorbed on a nonreflecting surface 28. The remaining portion of theenergy transmitted by the horn 16 is reflected from the sample 17 andreturns to the horn 16. A directional coupler 29 is connected betweenthe wave guide 16a and a wave guide 290 for coupling the reflectedenergy received by the horn 16 to the wave guide 29a The wave guide 29ais connected through the rectifier 21 to the integrator 22, which is inturn connected to the microwave indicator 19 which provides anindication of the reflected energy from the sample 17. Since thedielectric constant of the sample 17 varies with the moisture content,measurement of such reflected energy provides an indication of theamount of moisture content within the sample 17.

A difficulty encountered in applying the frequency modulation method tomeasuring moisture in a sample occurs when the moisture content isrelatively low, for instance below 30 percent. With high moisturecontent, an adequate signal to noise ratio is obtained when the horns 16and 18 are positioned perpendicular to the plane of the sample 17 andthe energy is passed through the sample only once. At low moisturecontents, such as 5 to l5 percent moisture, the attenuation attainedbypassing the energy once or even twice through the sheet is notadequate for practical use. It is possible to obtain a adequateattenuation level by passing the energy through the sheet several times.

Such an arrangement is illustrated in Figure 6, wherein the frequencymodulator is connected to the source 15, which is in turn connected tothe transmitting horn 16. It has been found that a minimum of six passesof the microwave energy through the sample 17 is necessary in order toachieve a usable signal to noise ratio. However, when the microwaveenergy is transmitted through the sample 17 a number of times, it wasfound that a high radiation loss occurred with each transmission of themicrowave energy from a separate horn. It was found that this difficultyis overcome by the use of dielectric microwave lenses which collimatethe microwave energy and reduce the losses to a tolerable level.

As shown in Figure 6, the microwave energy transmitted from the born 16and detected by a horn 30 passes through dielectric microwave lenses 31and 32 positioned on opposite sides of the sample 17 between the horns16 and 30. The horn 30 is connected by a wave guide to a transmittinghorn 33 wherein the microwave energy is retransmitted to the sample 17to a receiving horn 34. Dielectric microwave lenses 35 and 36 arepositioned on opposite sides of the sample 17 between the horns 33 and34. The receiving horn 34 is connected by means of a wave guide to atransmitting horn 37 which transmits the microwave energy through thesample 17 to the receiving horn 18 which is connected to the rectifier21 and the integrator 22 to the microwave indicator 19. Dielectricmicrowave lenses 38 and 39 are positioned on opposite sides of thesample 17 between the horns 37 and 18 to again collimate the microwaveenergy and reduce the losses resulting by the retransmission of themicrowave energy through the sample 17.

Another method of obtaining adequate attenuation level is that oftransmitting the microwave energy across the full width of the sample17. Such method as illustrated in Figures 7 and 8 of the drawingswherein the transmitting horn 16 is positioned at one edge of the sample17 and the receiving horn 18 is positioned at the opposite edge thereof.Although this method provides adequate attenuation level, the radiationlosses can be relatively high over the large distance of measurement.This problem of radiation losses, however, is overcome by the use ofmicrowave lenses 40 and 41 positioned at opposite edges of the sample 17and adjacent respective horns 16 and 18. The lenses 40 and 41 collimatethe energy and improve the efficiency of the transmission across thedistance of the sample 17 Transmitting the microwave energy across thefull width of the sample 17, however, can only provide an integratedmoisture content across the width of the sample and is not suitable formoisture profiling. It has been found, however, that the same conceptcan be employed while maintaining a capability of profiling of themoisture content across the width of the sample 17. This is accomplishedby traversing the microwave energy across the width of the sample 17 atrelatively shallow angles with respect thereto. Such method isillustrated in FIG. 9 wherein the transmitting horn 16 is posi' tionedadjacent to and below one edge of the sample 17 and the receiving horn18 is positioned adjacent to and above the opposite edge of the sample17. In order to reduce the radiation losses, microwaves lenses 42 and 43are positioned adjacent respective horns l6 and 18 for colliminating theenergy and improving the efficiency of the transmission across thedistance of the sample 17. The line of transmission and receptiondirection between the horns 16 and 18 defines a relatively shallow angledesignated with the reference numeral 44 with the plane of the sample17.

With the arrangement illustrated in FIG. 9, profiling is accomplished byraising and lowering the horns 16 and 18 as indicated by the arrowsadjacent thereto on the drawing in a manner which would maintainalignment of the microwave horns 16 and 18 and their radiated energy,but would tend to move the area of attenuation to different positions onthe sample 17.

FIG. 10 illustrates another embodiment of applicant's invention which isemployed for overcoming the low loss encountered in relatively drysamples under examination. In essence, the structure illustrated in FIG.10 is a travelling wave resonator. A typical description of a travellingwave resonator may be found in the I.R.E. transactions on microwavetheory and techniques, Apr. 1958, by L. J. Millsevic and R. Vautey onpage 136. A ring resonator is described in Microwave Engineering, A. F.Harvey pages 201-202. The travelling wave resonator, like the cavitydevices, might be employed to amplify low level attenuations.

As shown in FIG. 10, the modulated source of energy from the generator15 is transmitted through a wave guide 45 to a termination 46. The waveguide 45 is connected through a directional coupler 47 to a wave guide48 which is connected at one end thereof through a phase shifter 49 tothe transmitting horn 16 and at the opposite end thereof to thereceiving horn 18. The sample 17 is mounted between the horns 16 and 18and a pair of dielectric lenses 50 and 51 are disposed on opposite sidesthereof adjacent the horns l6 and 18. A directional coupler 52 isconnected between the .wave guide 48 adjacent the receiving horn l8 anda wave guide 53. The wave guide 53 is connected through the rectifier 21and integrator 22 to the microwave indicator 19.

An arrangement for changing the position of the sample 17 with respectto the horns 16 and 18 is illustrated in FIGS. 11 and 12 of thedrawings. As shown therein, the horn 16 is mounted on the support member54. A plurality of rails 55, 56 and 57 are supported between the supportmember 54 and a second support member 58. The support members 54 and 58are fixedly mounted with respect to one another. Movably mounted on therails 55, 56 and 57 is a first movable support member 59 which supportsthe sample 17 thereon and a second movable support member 60 whichsupports the receiving horn 18 thereon. The receiving horn 18 isconnected by means of a wave guide 61 to the crystal detector orrectifier 21. The arrangement illustrated in FIGS. 11 and 12 permitmovement of either the sample 17 or the receiving horn 18 with respectto the transmitting horn -16. If such movement is efi'ecte'd over adistance which is equal to one-half the wavelength of the microwaveenergy, the sensitivity due to sample position discussed above iseliminated.

The broad band frequency modulated microwave moisture measurement isapplicable to many products, such as paper,

textiles, foods, etc. In addition, however, all of the abo ve describedsystems for the measurement of moisture in a sample by the broad bandfrequency modulated technique can also be employed for measuringmoisture in a slurry. The technique employed by the present inventionfor measuring moisture in a slurry is that of extracting from a mainslurry line a portion of the slurry and distributing the contents intothe form of a sheet or web. This is accomplished by distributing theslurry in a relatively thin or narrow cavity provided in a material ofrelatively low loss dielectric.

FIGS. 13-16 illustrated a structural part which contains a relativelythin or narrow-cavity therein into which the slurry can be distributed.The illustrated structural part containing the cavity therein can besupported between the microwave horns l6 and 18 for determining themoisture content of the slurry. As shown in FIGS. 13-16, a slurrydistributor 65 is formed of a pair of plates 66 and 67 made of a lowloss dielectric material. A relatively wide and shallow channel 68 isformed inonesurface of the plate 67 which is joined by beveled portions69 and 70 to an inlet 71 and an outlet 72 respectively. Ribs 73 extendupwardly from a bottom surface of the channel 68 and serve to dispersethe slurry equally throughout the width of the channel 68. The plate 66also includes a pair of beveled portions 74 and 75 which join with theinlet 71 and the outlet 72 respectively. When the channeled surfaces ofthe plates 66 and 67 are abutted with one another relatively thin sheet.

I and the plates are secured together in such relationship, a relativelythin chamber 76 is formed between the inlet 71 and the outlet 72 fordispersing the slurry passing therethrough intoa It will be understoodthat variations and modifications may be effected without departing fromthe spirit and scope of the novel concepts of this invention. r

We claim: I

1. Apparatus for measuring the amount of moisture in a sample,comprising v t i a. a source of microwave energy,

b. a fixed frequency microwave energy radiating element connected tosaid source and positioned to impinge microwave energy on the sample, I

c. a microwave energy receiving element positioned to receive microwaveenergy emanating from the sample,

d. a microwave energy indicator connected to said receiving element,

e. means for continuously changing the transmission path from saidradiating element to the sample asmeasured in wavelengths of themicrowave energy by at least one-half I wavelength of the microwaveenergy, and wherein said means includes a disk of varying thickness ofhigh dielectric low loss material mounted between said radiating elementand the sample, and means for rotating said disk at a predetermined rateto present a continuously changing the transmission path through saiddisk to the microwave UNITED sTATEs PATENT OFFICE CERTIFICATE OFCORRECTION 3, 599,088 Dated August 10, 1971 Patent No.

Invent0r(s) Leroy H. Busker and Robert J. Mosher It is certified thaterror appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Column 2, line 70 through column 3, line 31, "such.. .least" shouldfollow Column 3, line 65, 17,

Column 5, line 12, (5 h ld d line 38, "bypassing" should read -bypassing-.

Column 6, line 26, "microwaves" should read -microwave.

Signed and sealed this 29th day of August 1972.

(SEAL) Attest:

EDWARD M.FLETCHER JR. ROBERT GOTTSCHALK At t es t ing Off icer Commissioner of Pa tent s FORM PO-1n50 [IO-69) LISCOMM-DC (GETS-P69 (T U 5LLOVFNNMI'NT "NINTING OF'HCF 959 O-366 334

1. Apparatus for measuring the amount of moisture in a sample,comprising a. a source of microwave energy, b. a fixed frequencymicrowave energy radiating element connected to said source andpositioned to impinge microwave energy on the sample, c. a microwaveenergy receiving element positioned to receive microwave energyemanating from the sample, d. a microwave energy indicator connected tosaid receiving element, e. means for continuously changing thetransmission path from said radiating element to the sample as measuredin wavelengths of the microwave energy by at least one-half wavelengthof the microwave energy, and wherein said means includes a disk ofvarying thickness of high dielectric low loss material mounted betweensaid radiating element and the sample, and means for rotating said diskat a predetermined rate to present a continuously changing thetransmission path through said disk to the microwave energy by one halfwavelength of the microwave energy.
 2. An apparatus as defined in claim1, further comprising a detector connected between said receivingelement and said indicator.
 3. An apparatus as defined in claim 2,further comprising an integrator connected between said detector andsaid indicator.